Modeling of radiative properties of an Oxyfuel atmosphere with a weighted sum of gray gases for variable carbon dioxide and water vapor concentrations

Abstract

The partial pressure of carbon dioxide in the flue gas of an Oxyfuel combustion process is significantly increased in comparison with conventional air-blown firing. Depending on the moisture content of the fuel and the type of flue gas recirculation (either wet or dry), the partial pressure of water vapor varies for Oxyfuel atmospheres. The calculation of the heat transfer by radiation in a furnace requires an accurate modeling of the optical properties of the flue gas. In order to reduce the computational effort in engineering calculations, the band radiation of the gaseous combustion products is approximated as a weighted sum of one clear and one or more gray gases. The partial pressures of carbon dioxide and water vapor of an Oxyfuel atmosphere exceed the range of published weighting factors and absorption coefficients. These have been developed for air-blown combustion with a high concentration of non-radiating nitrogen in the flue gas. New parameters for a weighted sum of one clear and four gray gases were determined in order to allow for the higher concentration of carbon dioxide in the flue gas. A fixed ratio between carbon dioxide and water vapor is no longer suitable for the calculation of the gas emissivity. Therefore, polynomials of the molar ratio of both radiating flue gas species represent the coefficients of the modified model. The emissivities calculated by this model are compared to emissivity data generated by the exponential wide band model. The heat transfer by radiation for a simplified, exemplary furnace is calculated for relevant atmospheres by computational fluid dynamics (CFD) software FLUENT using different models. The results show the suitability of the new model parameters to calculate the gas emissivity of Oxyfuel atmospheres with variable fractions of carbon dioxide and water vapor.